The Petrographic study in the ancient mortars characterization

The aim of this work is to underline the importance of the mineralogicalpetrographical study performed by observation of thin section under optical polarized microscope for the characterisation of mortars. The study of mortars is very interesting and not always easy. In fact it is not often possible an accurate characterization of binder and aggregate separately because of the difficulty to divide them. Therefore the petrographical study under polarized light, makes it possible to realize precise descriptions. As regards to the aggregate it is possible to determine the mineralogical composition (information on the material’s provenance); the grain size and shape of grains (information on sieving and grinding); the distribution inside the binder (information on mixing); the orientation of grains (indications on the pressure applied during the installation of a plaster). As regards to the binder it is possible to determine the following aspects: composition (presence of lumps or underburnt rock fragments suggesting the original carbonatic rock used in the production of the lime); presence of crystalline phases characteristic of some particular kinds of binder (i.e. hydromagnesite for magnesium lime); kind of porosity (primary or/and secondary) indicating the amount of water used for mixing or problems in the setting caused by a too rapid drying; the ratio binder/aggregate (which indicate a fat or lean mortar); presence of re-crystallization phenomena in the binder; state of conservation of the mixtures. In the paper several examples of mortars from different Italian regions and of different historic periods are showed in order to emphasise the importance of the pethographic observations in the study of this kind of material, the mortar, at present misunderstood and therefore so often considered not worthy to be conserved or even studied. KEY-WORDS: aggregate, binder, mortar